The icosahedral viruses of fungi are the largest and most widespread group of viruses with genomes of dsRNA. While most are latent in their hosts, those of Saccharomyces cereviseae and Ustilago maydis code for the production of a polypeptide toxin which diffuses throughout culture media and kills susceptible, related fungal strains but does not affect resistant (neutral) strains. In both species the killer strains and resistant strains contain cytoplasmically transmitted dsRNA viruses which apparently code separately for the toxin production and resistance. This investigation focuses on the relationship between viruses in killer strains and resistant strains of Ustilago maydis, through a study of their comparative physicochemical properties and serological relationships. Attempts will be made to positively identify the specific dsRNA segments (these viruses have segmented dsRNA genomes) responsible for toxin and viral capsid production by using denatured dsRNA segments as mRNA in a cell-free protein synthesizing system followed by the analysis of the polypeptides produced. Because these viruses infect organisms which are present as contaminants of food and because they contain genomes of dsRNA, a natural interferon inducer, they have importance to health.